1. Field of the Invention
The present invention relates to a wavelength detecting device and, more particularly, to a wavelength detecting device used for a wavelength multiplexing optical communication system to strictly control wavelengths, of signal beams, that are close to one another.
2. Description of the Related Art
Presently developed wavelength multiplexing communication systems multiplex 8 to 32 signal beams whose wavelengths are close to one another, e.g., between 0.8 nm (100 GHz) and 1.6 nm (200 GHz) apart.
For these signal beams, it is necessary to strictly control the wavelengths thereof. To handle signal beams having wavelength gaps of 0.8 nm, it is necessary to set a pass band of 0.2 nm for multiplexers and demultiplexers and a wavelength fluctuation of 0.1 nm for signal beams. If the wavelength of a signal beam shifts more than 0.1 nm, crosstalk will occur between the signal beam and adjacent signal beams. To prevent this, the communication system generally has a mechanism for fixing the wavelengths of signal beams.
If the wavelength of a signal beam in one channel excessively fluctuates, it will interfere with an adjacent channel to hinder the transmission of signal beams in both the channels. To prevent this, the communication system usually has, at the signal output side thereof, a wavelength selecting function to stop a signal beam if the wavelength thereof fluctuates extremely.
FIG. 1 shows a wavelength detecting device according to a prior art.
A laser diode 10 emits a signal beam having a wavelength of .lambda.. The signal beam is passed through a collimator lens 11 to form a collimated beam, which is passed through a collimator lens 15 to an optical fiber 16. Between the collimator lenses 11 and 15, there are arranged couplers 12 and 13 and a band-pass filter 14 having a pass wavelength of .lambda.. The band-pass filter 14 is arranged at an output port to block an output signal beam if its wavelength fluctuates extremely.
Part of the signal beam from the collimator lens 11 is branched by the couplers 12 and 13. The branched beams are passed through band-pass filters 17 and 18 having pass wavelengths of ".lambda.-.alpha." and ".lambda.+.alpha." to collimator lenses 19 and 20. The collimator lenses 19 and 20 focus the branched beams having the wavelengths ".lambda.-.alpha." and ".lambda.+.alpha." on photodiodes 21 and 22, which convert the beams into electric signals.
The output currents of the photodiodes 21 and 22 are supplied to a differential amplifier 23 and a level meter 24 to detect a zero point where the current difference between the photodiodes 21 and 22 becomes zero. The zero point corresponds to a cross point of wavelength characteristic curves around the wavelengths ".lambda.-.alpha." and ".lambda.+.alpha." and is equal to the center wavelength .lambda. of the signal beam.
According to the output of the level meter 24, a control circuit (not shown) carries out feedback control on the wavelength of the signal beam emitted from the laser diode 10 as indicated with a dotted line, to maintain the output of the level meter 24 at zero, i.e., to maintain the wavelength of the signal beam at .lambda..
This prior art employs two couplers 12 and 13 in a signal beam path, to increase the signal loss. In addition, the prior art must arrange three wavelength selecting elements, i.e., the band-pass filters 14, 17, and 18. These elements increase the number of components in the wavelength detecting device, need individual adjustments and increase the manufacturing cost.